1. Field of the Invention
This invention relates to apparatus for scanning opaque documents to provide an electrical signal which corresponds to the information on the documents.
2. Description of Prior Art
Various arrangements are known in the art for converting information contained on a surface to a corresponding electrical signal. Such arrangements generally involve two basic functions, that is, deflecting a light beam across the information and detecting the amount of light which is either transmitted through the surface if the surface is transparent, or detecting the amount of light reflected back from the surface if the surface is opaque. In those arrangements where the detector is sensing the amount of light transmitted through the document, the optical path from the light source to the detector is generally less complex than where the detector is sensing reflected light. In the latter situation, both the detector and the light source must be positioned on the same side of the surface -- and since these are physical components, the path from the light source to the document must necessarily be different than the path for the reflected light from the document to the detector. The amount of light energy reflected from a surface is of course dependent upon the amount of light from the source and the reflective characteristics of the surface. If the surface to be scanned is an ordinary document, the percentage of energy reflected is relatively low. The cost of the light source is generally directly proportional to the energy that it can deliver while the cost of a light-responsive detector is also generally proportional to its sensitivity. The power of the light source, the reflectivity of the surface and the sensitivity of the detector must all be considered in the design of any practical opaque document scanner. In addition, for a given surface reflectance, cost tradeoffs exist between the type of light source and the type of detector.
Laser light sources have been recognized as efficient, economical, high-energy light sources for document scanning applications. However, until recently, the arrangements proposed for achieving the scanning function have been relatively cumbersome and costly. This situation was caused by the need to provide in the light path complicated lens systems and moving components such as multi-faceted rotating mirrors which would cause the beam to move in some predetermined scan direction across the document. The number of scan lines per inch, i.e., resolution, required to convert the information on the surface into electrical signals determined the allowable tolerance for the moving components which, in turn, directly affected the cost. In addition, where the optical path for the beam deflector involved many mechanical components and the information on the document was sensed by the amount of light reflected, the path for the reflected light from the document generally also required an optical component whose movement had to be maintained in synchronism with the moving component of the scanning system. The manner in which the synchronization was achieved also affected the cost of such system.
The use of holograms in document scanning systems has been demonstrated in the prior art. It has been shown, for example, in U.S. Pat. No. 3,795,768, that a reflection hologram which is rotated can produce a scanning circle composed of a locus of points of radiation when the hologram is illuminated with a beam of light from a laser source. In such a system the document to be scanned is positioned on a segment of the locus and a single line scan is achieved. In this system, the document is moved parallel to the axis of rotation of the hologram so that the entire document may be scanned.
Another type of arrangement is discussed in Applied Optics 1967, Vol. 6, No. 9 pages 1531-1534. In this system a transmissive hologram is rotated between the laser light beam and the document and the image spot follows a circular path in the plane of the document. Such a system is generally not suitable where a linear scan line is required,
In our co-pending application referred to above, there is disclosed a holographic deflector for a laser light beam in which scan lines are provided across a document. The structure of the hologram in that application provides a scanning line which is not parallel to the direction of movement of the hologram. A more conventional line scan, for example, one which would scan across the width of a printed page, can be generated by the device disclosed in an article entitled "Holographic Laser Beam Deflector" by Pole and Wollenmann published on Apr. 4, 1975, in Applied Optics Vol. 14, No. 4, beginning on page 976. The present invention employs some structure similar to that disclosed in the above-mentioned paper.